Solar cells are photovoltaic components for direct generation of electrical current from sunlight. A number of solar cells are provided on a substrate that may be referred to as a solar cell substrate, a solar panel, or a solar module. The solar cell substrate captures energy from sunlight. Photons in sunlight hit the solar cells and are absorbed by semiconductor materials such as Cu(In,Ga,)Se2 (CIGS), (silicon, or other absorber materials. Negatively charged electrons are released from their atoms by the photons, causing an electric potential difference. Current starts flowing through the solar cell material to cancel the potential difference and this electricity is captured. The electricity produced by a multitude of solar cells on the solar cell substrate is harnessed and coupled to a power cable.
A variety of solar energy collecting modules currently exists. The solar energy collecting modules can have different geometries and be formed of different materials, but generally include large, flat solar panels and include a back contact layer, an absorber layer, a buffer layer and a front contact layer. The buffer layer can comprise a material such as CdS, and is used to produce a reproducible and efficient heterojunction in thin film solar cells. CdS buffer layers also find application in other types of solar cells.
Cu(In,Ga)Se2 (CIGS) type solar cells. CIGS solar cells use a CIGS absorber layer, and buffer layer (e.g., CdS) disposed between the absorber layer and a ZnO window layer. Integrating a CdS layer into the ZnO/CIGS system enhances the spectral absorption of the solar cell, since the refractive index of CdS (nr˜2.4) lies between the refractive indices of ZnO (nr˜1.9) and CIGS (nr˜2.9). The large step between the refractive indices of ZnO and CIGS is therefore divided into two smaller steps resulting in an overall reduced reflectivity of the solar cell. Thus, the quality of the buffer layer affects in overall solar cell performance.